Thermoplastic resin films are popularly used as household wraps and bags to contain food and other items. A necessary characteristic of such wrap films is the ability to "cling", that is, to adhere to itself or to form a tight seal with other surfaces, as when the film is used as a cover for containers. The clingability of the film preserves the freshness and moisture content of wrapped food items and also prevents the migration of food odors.
Thermoplastic resin films do not generally possess inherent cling characteristics sufficient to satisfactorily adhere to themselves and other surfaces. The cling characteristic of thermoplastic films is generally obtained through the use of so-called cling agents. Cling agents are chosen for their ability to produce a surface on a thermoplastic film that can be sealed, opened and resealed, and are selected with due care in consideration of the use of thermoplastic films in food contact applications.
With the increased use of microwave ovens, it has become desirable for food cling wraps to be used in microwave cooking. Currently, commercial food wraps such as Saran Wrap.RTM., Handi-Wrap.RTM., and Glad.RTM. Wrap are produced as a single layer film structure that is modified with additives to provide the desired "cling" property. These commercial cling wraps are useful in microwave cooking as a cover for microwave cooking utensils and, as such, are not in contact with the food with which they are to be used. Functionally, it would be desirable to have a food wrap which withstands higher temperatures so that it could be used in direct food contact, and in essence, serve the purpose of a microwave cooking utensil. That is, food would be wrapped in the food wrap, frozen, and when needed the food could be cooked in a microwave oven in the original food wrap.
Commercial food cling wraps may be produced from various polymers such as low density polyethylene, polyvinylidene chloride copolymers, and polyvinyl chloride copolymers. Currently, only polyvinylidene chloride and polyvinyl chloride type food cling wraps are able to withstand somewhat higher microwave oven cooking temperatures, that is, on the order of up to about 335.degree. F. However, at these temperatures, the polyvinylidene chloride and polyvinyl chloride type food wraps have limited use for direct food contact microwave cooking since many fatty foods and foods high in sugar content require higher cooking temperatures than such food wraps can withstand without degradation.
In addition, in the production of plastic films for use as food wraps, a balance must be attained between the film's "cling" and "handleability" characteristics. "Handleability" is the characteristic of the film to resist the tangling that results from the film clinging to itself during use. While cling agents impart good cling properties to the film, it has been found that an increase in the amount of cling agent that is incorporated into the film corresponds to a decrease in the handleability of the film. With increased cling agent, the tendency of the film to adhere to itself and tangle increases.
It is known that handleability is improved through the use of "antiblocking" agents. "Antiblocking" agents prevent thin plastic films from sticking to one another. Antiblocking agents suitable for use in plastic films include, but are not limited to, diatomaceous silica, such as those commercially available under the tradenames "SUPERFLOSS", "SILVERFROST", and "CELITE" which are silica products of the Johns-Manville Company. The concentration of the antiblocking agent incorporated into the film generally ranges from about 100 to about 10,000 parts per million, and usually from about 3,000 to about 7,000 parts per million. As disclosed in U.S. Pat. No. 3,028,355, the diatomaceous silica, referred to as diatomaecous earth in the patent, should generally have an average particle size of between about 2 to about 6 microns. Other antiblocking agents selected by those skilled in the art can also be suitably used.
In addition to having good "cling" and "handleability" characteristics", as well as having a resistance to heat for use in direct food contact microwave cooking, the food wrap should have good barrier properties such as oxygen, moisture, and food odor impermeability. Further, the food wrap should possess a degree of toughness. All of these properties cannot be found in a single layer film food wrap. It is only by providing a multilayer film that all of the aforementioned desirable properties and characteristics may be attained in a food wrap. Multilayer films consist of two or more layers, each imparting specific desired characteristics to the multilayer film.